I have the same situation, ~20k @ 2.25-2.50% interest, cosigned by my parents. Chances are if you drop the cosigner, your rates will increase.
I chose to invest everything last year instead of paying this off, and made mad money in the market. Sure that won't happen every year, and yes there's a chance to lose money. I can always cash out some investments if I had to pay it off fast. Your net-worth will quickly grow past 13k, so it isn't a significant risk to the cosigner. Not to mention the extra tax advantages from maxing 401k/IRA's is already better than 2.5% interest. Whenever I think about doing something stupid and getting into debt, I have my student loan to look at as a nice deterrent.
However, with 200$/month payments, it will take a lifetime to be free. So at some point you just have to get rid of it. My plan is to use cash windfalls, bonuses & tax returns, to pay it down faster.
My priorities were high interest (6-7% stafford) loans first, max 401k/IRA, taxable account, remaining 2% loans.